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KVO.md

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一、KVO简介

1. KVO

KVO的全称是Key-Value Observing，俗称“键值监听”，可以用于监听某个对象属性值的改变

2. KVO的使用

  • 发起监听

    • 可以通过addObserver: forKeyPath:方法对属性发起监听

  • 接收监听信息

    • 然后通过observeValueForKeyPath: ofObject: change:方法中对应进行监听，见下面示例代码:

    // 示例代码 @interface Person : NSObject

    @property (assign, nonatomic) int age; @property (assign, nonatomic) int height; @end

    @implementation Person

    @end

    @interface ViewController ()

    @property (strong, nonatomic) Person *person1; @property (strong, nonatomic) Person *person2; @end

    @implementation ViewController

    • (void)viewDidLoad { [super viewDidLoad];

      self.person1 = [[Person alloc] init]; self.person1.age = 1;

      self.person2 = [[Person alloc] init]; self.person2.age = 2;

      // 打印添加监听之前person1和person2对应的isa指针指向的类型 NSLog(@"person1添加KVO监听之前 - %@ %@", object_getClass(self.person1), object_getClass(self.person2)); // 打印结果：Person Person

      // 打印添加监听之前person1和person2对应的setAge方法是否有改变 NSLog(@"person1添加KVO监听之前 - %p %p", [self.person1 methodForSelector:@selector(setAge:)], [self.person2 methodForSelector:@selector(setAge:)]); // 0x10b60c4b0 0x10b60c4b0

      // 给person1对象添加KVO监听 NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld; [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];

      // 打印添加监听之后person1和person2对应的isa指针指向的类型 NSLog(@"person1添加KVO监听之后 - %@ %@", object_getClass(self.person1), object_getClass(self.person2)); // 打印结果：NSKVONotifying_Person Person

      // 打印添加监听之后person1和person2对应的setAge方法是否有改变 NSLog(@"person1添加KVO监听之前 - %p %p", [self.person1 methodForSelector:@selector(setAge:)], [self.person2 methodForSelector:@selector(setAge:)]); // 0x7fff207b62b7 0x10b60c4b0 }

    • (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event { self.person1.age = 20;
      }

    • (void)dealloc { [self.person1 removeObserver:self forKeyPath:@"age"]; }

    // 当监听对象的属性值发生改变时，就会调用

    • (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context { NSLog(@"监听到%@的%@属性值改变了 - %@ - %@", object, keyPath, change, context); }

    @end 复制代码

注意：  监听的对象销毁之前要移除该监听removeObserver: forKeyPath:

二、KVO的实现本质

1.通过上面示例代码发现，函数在调用addObserver: forKeyPath:方法之后，person1的实例对象的isa指针指向了一个新的类型NSKVONotifying_Person，而没有添加监听的person2的isa指针还是指向了Person这个类型

2.我们发现通过object_getClass打印person1的类对象和元类对象都是新派生出来的NSKVONotifying_Person这个类型

NSLog(@"类对象 - %@ %@",
          object_getClass(self.person1), 
          object_getClass(self.person2)); 
// NSKVONotifying_Person Person

NSLog(@"元类对象 - %@ %@",
          object_getClass(object_getClass(self.person1)), 
          object_getClass(object_getClass(self.person2))); 
// NSKVONotifying_Person Person
 
复制代码

3.我们发现通过object_getClass打印person1的superclass是Person这个类型，说明新派生出来的NSKVONotifying_Person是Person的子类

NSLog(@"父类 - %@ %@", 
		object_getClass(self.person1).superclass,
        object_getClass(self.person2).superclass);
        // Person NSObject
复制代码

4.通过打印我们发现，person1调用的setAge方法的内存地址发生了改变，通过LLDB打印该地址的详细信息发现setAge方法的实现实际是Foundation框架中的_NSSetIntValueAndNotify这个函数

(lldb) p (IMP)0x7fff207b62b7
(IMP) $2 = 0x00007fff207b62b7 (Foundation`_NSSetIntValueAndNotify)
(lldb) p (IMP) 0x108801480
(IMP) $3 = 0x0000000108801480 (Interview01`-[Person setAge:] at Person.m:13)
复制代码

5.我们手动创建这个派生类型NSKVONotifying_Person，并且在Person里面重写setAge:、willChangeValueForKey:、didChangeValueForKey:这三个方法，运行程序并观察调用情况

@interface NSKVONotifying_Person : Person

@end

@implementation NSKVONotifying_Person

@end


@interface Person : NSObject

@property (assign, nonatomic) int age;
@property (assign, nonatomic) int height;
@end

@implementation Person

- (void)setAge:(int)age
{
    _age = age;
    
    NSLog(@"setAge:");
}

- (void)willChangeValueForKey:(NSString *)key
{
    [super willChangeValueForKey:key];
    
    NSLog(@"willChangeValueForKey");
}

- (void)didChangeValueForKey:(NSString *)key
{
    NSLog(@"didChangeValueForKey - begin");
    
    [super didChangeValueForKey:key];
    
    NSLog(@"didChangeValueForKey - end");
}

@end
复制代码

由此可见，当监听的属性发生改变，系统派生出的这个类NSKVONotifying_Person(在)会对应的先后调用

  • willChangeValueForKey:

  • setAge:

  • didChangeValueForKey: 这三个方法，并在didChangeValueForKey:里调用观察者的observeValueForKeyPath: ofObject: change:来通知值属性值的变化

    // 执行后打印 2021-01-19 13:42:02.071987+0800 Interview01[37119:19609444] willChangeValueForKey 2021-01-19 13:42:02.072192+0800 Interview01[37119:19609444] setAge: 2021-01-19 13:42:02.072332+0800 Interview01[37119:19609444] didChangeValueForKey - begin 2021-01-19 13:42:02.072662+0800 Interview01[37119:19609444] 监听到<Person: 0x6000036ac2c0>的age属性值改变了 - { kind = 1; new = 21; old = 1; } - 123 2021-01-19 13:42:02.072817+0800 Interview01[37119:19609444] didChangeValueForKey - end 复制代码

6.通过class方法打印person1的类发现还是Person这个类型，说明在派生出的这个类NSKVONotifying_Person内部重写了class方法，并返回的是Person这个类型。所以只能通过object_getClass才能获取到真实的类型

NSLog(@"%@ %@",
          [self.person1 class], 
          [self.person2 class]); 
// Person Person

NSLog(@"%@ %@",
          object_getClass(self.person1), 
          object_getClass(self.person2)); 
// NSKVONotifying_Person Person
复制代码

7.通过Runtime的class_copyMethodList函数查看NSKVONotifying_Person内部还动态生成了dealloc、_isKVOA这两个函数

- (void)printMethodNamesOfClass:(Class)cls
{
    unsigned int count;
    // 获得方法数组
    Method *methodList = class_copyMethodList(cls, &count);
    
    // 存储方法名
    NSMutableString *methodNames = [NSMutableString string];
    
    // 遍历所有的方法
    for (int i = 0; i < count; i++) {
        // 获得方法
        Method method = methodList[i];
        // 获得方法名
        NSString *methodName = NSStringFromSelector(method_getName(method));
        // 拼接方法名
        [methodNames appendString:methodName];
        [methodNames appendString:@", "];
    }
    
    // 释放
    free(methodList);
    
    // 打印方法名
    NSLog(@"%@ %@", cls, methodNames);
}

[self printMethodNamesOfClass:object_getClass(self.person1)];
[self printMethodNamesOfClass:object_getClass(self.person2)];

// 打印结果
2021-01-19 15:38:13.552990+0800 Interview01[41940:19730538] NSKVONotifying_MJPerson setAge:, class, dealloc, _isKVOA,
2021-01-19 15:38:13.553166+0800 Interview01[41940:19730538] MJPerson setAge:, age,
复制代码

三、总结

通过上面一系列操作可以汇总为：

  • 利用RuntimeAPI动态生成一个子类，并且让instance对象的isa指向这个全新的子类
  • 全新的子类会重写class这个函数，并返回父类类型
  • 在全新的子类里面会重写被监听的成员对象/属性的setter方法,当修改instance对象的属性时，会调用Foundation的_NSSetXXXValueAndNotify函数
    • 函数内部首先调用willChangeValueForKey:
    • 紧接着函数内部 调用父类原来的setter
    • 最后调用didChangeValueForKey:
      • 内部会触发监听器（Oberser）的监听方法 observeValueForKeyPath:ofObject:change:context:

四、KVO的应用场景

  • 1.监听ScrollView的偏移量，改变导航栏背景色
  • 2.给TextView增加placeHolder，通过KVO监听文本是否输入对应隐藏展示placeHolder
  • ...